Asymmetries in topographic brain maps of auditory evoked potentials in the elderly.

We compared topographic brain maps of the middle response (MR) and late response (LR) auditory evoked potentials in two groups of elderly males. Subjects were categorized on the basis of the performance of the poorer hearing ear on the Synthetic Sentence Identification (SSI) test. Evoked potentials, obtained in response to 1000-Hz tone pips and tone bursts, were recorded under three conditions of sound stimulation:binaural (BIN), left ear only (LE), and right ear only (RE). Comparisons of the topographic brain maps generated by the two groups revealed significant asymmetry in the MR of the experimental group. LE stimulation produced a larger response over the right hemisphere, and RE stimulation produced a larger response over the left hemisphere. This asymmetry was not observed in the experimental group with binaural stimulation, or in the control group in any of the three stimulation conditions. An analogous asymmetry was not recognized in the LR in either group for any of the three sound conditions. Results are interpreted in relation to the recently proposed model of multiple middle latency generators.